Bug 1493707 - bug selecting a closed parent tab (#4259)

* Bug 1493707 - bug selecting a closed parent tab

* add another state assertion

Author: garvankeeley

Client/Frontend/Browser/Tab.swift

@@ -134,7 +134,7 @@ class Tab: NSObject {
     var screenshotUUID: UUID?
 
     // If this tab has been opened from another, its parent will point to the tab from which it was opened
-    var parent: Tab?
+    weak var parent: Tab?
 
     fileprivate var contentScriptManager = TabContentScriptManager()
     private(set) var userScriptManager: UserScriptManager?

Client/Frontend/Browser/TabManager.swift

@@ -194,6 +194,7 @@ class TabManager: NSObject {
         } else {
             _selectedIndex = -1
         }
+        assert(_selectedIndex > -1, "Tab expected to be in `tabs`")
 
         store.preserveTabs(tabs, selectedTab: selectedTab)
 
@@ -439,7 +440,7 @@ class TabManager: NSObject {
     // Select the most recently visited tab, IFF it is also the parent tab of the closed tab.
     func selectParentTab(afterRemoving tab: Tab) -> Bool {
         let viableTabs = tab.isPrivate ? privateTabs : normalTabs
-        guard let parentTab = tab.parent, parentTab != tab, !viableTabs.isEmpty else { return false }
+        guard let parentTab = tab.parent, parentTab != tab, !viableTabs.isEmpty, viableTabs.contains(parentTab) else { return false }
 
         var parentTabIsMostRecentUsed = true
         for candidate in viableTabs {